Assembly of a large number of semiconductor structural elements requires the production of wire bridges between the contact islands (bond pads) on the semiconductor chips and points of contact on external connections. If wire bridges are to be made using aluminum wire, the generally known ultrasonic wire bonding process is preferred for bonding.
Since modern semiconductor structural elements often require a large number of bonds, ultrasonic wire bonding preferably is performed by automatic ultrasonic wire bonders. Because of the large volume involved, special importance is of course attached to productivity. This means that wire bridges must be produced within as short as possible a time while maintaining the quality of the resulting bondings. Bonding connections between the bonding wire and the particular point of contact must have sufficient adhesive force, i.e., the elements to be welded together must have entered into a metallic bond with sufficiently great contact surface.
In order to ensure proper bond strength, a variety of parameters need to be considered. Such parameters include bonding force, ultrasonic energy or alternatively the level of ultrasonic vibrations and the duration of action of ultrasonic vibrations. These parameters may be specified by means of a suitable control device. Additional parameters that directly influence the welding process are material parameters which relate to the elements to be welded together (bonding wire, point of contact).
It has been found in previously known quality control methods that individual parameters were recognized as essential and were then selectively monitored during the ultrasonic welding process.
One of these parameters is the deformation of the bonding wire during the ultrasonic welding process. For example, EP 0 208 310 A1 discloses a method having a device suitable for controlling the course of the process and for quality control in ultrasonic welding of work pieces. In this case, the deformation of the work pieces to be welded together is utilized for controlling process variables. To this end, a displacement transducer in the form of a laser measuring device is provided for measuring the position of the ultrasonic sonotrode (bond wedge). Here the ultrasonic generator is cut off when a specific wire deformation is reached.
A similar accomplishment has been disclosed in EP 0 540 189 B1. In this case, quality control is effected by monitoring the deformation of the wire, and controlling the duration and level of ultrasonic action accordingly. The level is controlled such that the level is inversely proportional to bond wire deformation, i.e., ultrasound level is reduced with increasing deformation.
A wire connection or wiring device having an ultrasonic modulator has also been disclosed in DE 33 33 601 A1, wherein the deformation of the wire is monitored by means of a slot sensing device. Here the amplitude and the duration of ultrasonic action is controlled as a function of the deformation of the wire. In addition, a contact control, by which the variation in impedance in the contact time is controlled, is also provided.
Another method for controlling ultrasonic welding processes has been disclosed in DD 217 454 A1, wherein for quality control purposes, a continuous impedance measurement takes place even during subsequent ultrasonic action on the wire so that the contact time can be determined. Since the impedance continues to vary during ultrasonic bonding, the measured values can constantly be compared with desired values. As soon as the desired value representative of the cutoff time is reached, the ultrasonic generator is cut off.
However, it has been found that quality control by the above methods or devices is insufficient and has many shortcomings. Monitoring the deformation of the wire is not sufficient, since this parameter does not guarantee that a welded connection with the desired properties has actually been produced upon cutoff of the ultrasonic vibrations when a specific deformation is reached. For example, if the bonding process is started with too much energy, rapid deformation indeed takes place, but a metallic bond between the elements to be welded together may fail to form even though the value of maximum deformation has been reached. Similar problems also arise when the amplitude/output of the ultrasonic vibrations is varied during the welding process. Accordingly, spontaneous strong deformation of the wire should be prevented.